1. Field of the Invention
The present invention relates to an image forming apparatus such as an electro-photographic copying machine and printer including an intermediate transfer belt.
2. Description of the Related Art
Recently, an intermediate transfer belt type image forming apparatus has been known which primarily transfers a toner image formed on a photosensitive drum (image carrier) onto an intermediate transfer belt suspended by a plurality of tension rollers to secondarily transfer the toner image on the intermediate transfer belt onto a recording material. This intermediate transfer belt type image forming apparatus transfers the toner image formed on the image carrier in a lump. Thus, the intermediate transfer belt type image forming apparatus has the advantage of high suitability for the recording material. Further, the intermediate transfer belt type image forming apparatus has a structure for tensing an intermediate transfer belt by a plurality of tension rollers. Thus, intermediate transfer belt type image forming apparatus has the advantage of relatively freely stretching and rotating a belt, and allowing an image forming apparatus to be miniaturized.
However, the following problem will arise when this intermediate transfer belt is adopted as a high-speed image forming apparatus and a miniaturized image forming apparatus.
Time that the intermediate transfer belt electrically charged in primary transfer takes to move from a primary transfer portion to a secondary transfer unit, is short. Thus, an electric charge retained in the intermediate transfer belt cannot sufficiently be attenuated. This will cause a portion of the intermediate transfer belt, which cannot be attenuated, to come close to the grounded tension roller for tensing the intermediate transfer belt.
On the other hand, in such a configuration, as the intermediate transfer belt electrically charged in the primary transfer draws apart from the image carrier and the transfer member, the intermediate transfer belt is led to a high electric potential. As a result of this, an electric potential difference between the intermediate transfer belt and the tension roller or members in the vicinity thereof becomes large and generates an electric discharge in a minute void part.
When the electric discharge is generated, a phenomenon occurs in which a toner on the intermediate transfer belt is electrically charged with a polarity reverse to a normal electric-charge polarity. Thus, a problem arises in which the toner electrically charged with a reverse polarity is not transferred onto the recording material in the secondary transfer.
This phenomenon will be described using FIG. 7.
First, an electric potential difference between an intermediate transfer belt and a tension roller needs to receive attention. Previously, an electric potential Vitb of the intermediate transfer belt after primary transfer can be represented by the following equation:Vitb=(Qitb+CrollVroll+CdrumVdrum)/(Croll+Cdrum)  (1)where Vitb denotes an electric potential of the intermediate transfer belt, Vroll denotes a surface electric potential of primary transfer, Vdrum denotes a surface electric potential of an image carrier, Qitb denotes the amount of an electric charge accumulated in the intermediate transfer belt, Croll denotes an electric capacity between the intermediate transfer belt and a primary transfer roll, and Cdrum denotes an electric capacity between the intermediate transfer belt and a photosensitive drum. The surface resistivity of the intermediate transfer belt is 1010Ω/□ to 1013Ω/□ and the volume resistivity of the intermediate transfer belt is 107 Ω·cm to 1012 Ω·cm.
In a configuration in which the attenuation of the amount of an electric charge Qitb is small, an electric potential difference between the intermediate transfer belt and the tension roller is changed by an electric capacity between a belt and a drum, and an electric capacity between a belt and a primary transfer roller.
As an example, a high-speed image forming apparatus in which a travel speed of an intermediate transfer belt is 500 mm/sec. will be described. With respect to the amount of an electric charge Qitb accumulated in the intermediate transfer belt, self attenuation of an electric charge of a belt does not sufficiently occur until the belt reaches a downstream tension roller as the belt travels downstream.
On the other hand, respective electric capacities Croll and Cdrum are inversely proportional to a distance between a belt and a drum, and a distance between a belt and a primary transfer roller. Thus, as the intermediate transfer belt travels, the respective electric capacities Croll and Cdrum decrease.
On the other hand, when a plurality of tension rollers is present, a part of an electric charge of the intermediate transfer belt will be eliminated by a tension roller which is first brought into contact therewith after primary transfer. However, in an intermediate transfer belt which travels at a high speed, it is difficult to eliminate an entire electric charge.
As a result of this, the electric potential of the intermediate transfer belt is increased with the above-described equation. Thus, an electric discharge will occur when a part having a large electric potential is brought into contact with the grounded another tension roller or passes through the vicinity of another member.
This electric discharge reverses a polarity of a toner in a portion receiving the electric discharge. Thus, the toner with the reversed polarity is not transferred in a secondary transfer portion. Hence, a white patch which is referred to as an electric discharge trace also occurs.
Thus, Japanese Patent Application Laid-Open No. 2002-82532 is configured to provide an electric charge elimination brush which comes into contact with the reverse side of the intermediate transfer belt to eliminate an electric charge, in the upstream part of the tension roller with which the intermediate transfer belt passing through the primary transfer portion first brings into contact. Further, Japanese Patent Application Laid-Open No. 2004-317915 is configured to dispose an electric charge elimination needle as an electric charge elimination unit.
On the other hand, as a method for controlling an electric potential of an intermediate transfer belt, Japanese Patent Application Laid-Open No. 2007-240750 is configured to dispose a sheet metal member arranged at a predetermined interval between itself and the intermediate transfer belt in a position opposite to the intermediate transfer belt. When such a configuration to eliminate an electric charge of the intermediate transfer belt is adopted, the following problem may arise depending on an electric charge elimination point. Even if the electric charge elimination brush is provided in a position more upstream than a position where the intermediate transfer belt first brings into contact with the tension roller, when the electric charge elimination brush is disposed in an area opposite to the metal member, an electric discharge may occur between the tension roller or the like on the downstream side and the intermediate transfer belt.
This is because the electric potential of the intermediate transfer belt is restricted by the metal member, an electric potential difference between the electric charge elimination member and the intermediate transfer belt is reduced, and an electric charge cannot be sufficiently eliminated. As a result of this, after the intermediate transfer belt passes through the image carrier and the metal member, the electric potential of the intermediate transfer belt increases again.
A relevant mechanism will be described using FIG. 8.
As illustrated in FIG. 8, an electric potential of an intermediate transfer belt after primary transfer can be represented by the following equation:Vitb=(Qitb+CrollVroll+CmVm)/(Croll+Cm)  (2)where Vitb denotes an electric potential of the intermediate transfer belt, Vroll denotes a surface electric potential of a primary transfer roller, Vm denotes a surface electric potential of a metal member, Qitb denotes the amount of an electric charge accumulated in the intermediate transfer belt, Croll denotes an electric capacity between the intermediate transfer belt and the primary transfer roll, and Cm denotes an electric capacity between the intermediate transfer belt and the metal member. The amount of an electric charge Qitb accumulated in the belt denotes the amount of a toner electric charge of a transferred toner and an electric charge supplied in the primary transfer portion, and is roughly constant similarly to the previous one.
Further, as illustrated in FIG. 8, in an area in which the intermediate transfer belt and the metal member are opposed to each other, a distance between the intermediate transfer belt and the metal member is roughly fixed. Thus, an increase in the electric potential of the intermediate transfer belt is limited. However, after passage of the metal member, the distance is gradually increased. Thus, an electric potential difference between the belt and the metal members starts to be increased.
Essentially, the belt electric potential is attenuated with time. However, when a travel speed of the intermediate transfer belt is fast, the intermediate transfer belt is brought into contact with the tension roller before the belt electric potential is sufficiently attenuated.
As a result of this, after the intermediate transfer belt passes through an area opposite to the metal member, the electric potential of the intermediate transfer belt is increased. Thus, an electric discharge phenomenon occurs in the tension roller or the like.
Thus, it is desirable to eliminate an electric charge in a part opposing the metal member in which an increase in the electric potential of the intermediate transfer belt is large.
On the other hand, in order to sufficiently eliminate an electric charge, in other words, eliminate an electric charge by an electric discharge in an area where an increase of such the electric potential is large, it is effective to use an electric charge elimination member which is not in contact with an object to be electrically discharged.